The present invention pertains to a method and system for improving delivery of a slurry across a face of a multilayered polishing pad and, more particularly, to polishing large scale assemblies, for example, those used in the manufacture of large area field emission display devices.
Field emission display (FED) technology, as represented for example by U. S. Pat. No. 5,210,472, the disclosure of which is incorporated herein by reference, utilizes a matrix addressable array of pointed, thin film, cold field emission cathodes in combination with a phosphor luminescent screen. The FED incorporates a column signal to activate a column switching driver and a row signal to activate a row switching driver. At the intersection of both an activated column and an activated row, a grid-to-emitter voltage differential exists sufficient to induce a field emission, thereby causing illumination of the associated phosphor of a pixel on the phosphorescent screen. Extensive research has recently been made into the manufacture of an inexpensive, low power, high resolution, high contrast, full color FED.
However, the known FEDs have several structural shortcomings; first and foremost has been their size. It has been possible to produce FEDs of rather small area, but it has theretofore been difficult to produce an FED of sufficient area, for example for the display of a laptop computer or a hand held portable television set. One of the problems in making larger FEDs is producing a uniform surface on the substrate. The present invention addresses this problem.
Fibrous polishing pads per se are well known. For example, U.S. Pat. Nos. 4,728,552; 4,841,680; 4,927,432; and 4,728,552, each disclosure of which is incorporated herein by reference, all describe poromeric pads made from microporous materials. Likewise polishing slurries are known; see for example U.S. Pat. Nos. 4,959,113; 5,264,010; 5,382,272; 5,389,352; and 5,391,258, the disclosures of which are also incorporated herein by reference. A suitable apparatus for planarization of large area substrates for use in field emission displays, and which would benefit from the present invention, is described in U.S. Pat. No. 5,257,478, the disclosure of which is also incorporated herein by reference.
Microchannelling in polishing is a problem that can cause the leading edge of substrates to bind and break. This is true with substrates which have more than one leading edge transition. It becomes even more important when the area to be polished increases and the tolerances decrease. For example, in large area field emission displays, it is important that the substrates be microscopically flat, which means that the surface will have undulations in the range of between about 0.1 and about 4.0 microns from a median plane of the surface. Conventional slurry delivery is concentrated on the edges of a pad without much concern for uniform delivery of the slurry across the face of the surface to be polished. Slurry delivery to the center of larger substrates, of the type contemplated for use in large area field emission display devices, is necessary to accomplish uniform polishing and finish. Thus, there is a need for a method and device to provide for improved polishing and controlled delivery of slurry. It is an object of the present invention to address these needs.